Insulin is secreted from beta cells of pancreatic Langerhans islet in its active form. The human insulin is composed of two polypeptides, the A and B chain, usually of 21 and 30 amino acids residues, respectively, with a molecular mass of about 5800 DA. The peptides are interconnected by disulfide bonds of the cysteine residues at A7-B7, A20-B19 and A6-A11. Insulin exerts a wide variety of biological activities including controlling the uptake, utilization, and storage of cellular nutrients such as glucose, amino acids and fatty acids. The important target tissues of insulin are liver, muscle and fat but many other cell types are also influenced by this hormone. [Davis, S N; Granner, D K (1996) In: Goodman & Gilman's The Pharmacological Basis of Therapeutics. ninth edition ed. (Eds: Hardman, J G et al.) McGraw-Hill, 1487--1517].
Diabetes and Insulin: One of the main physiological roles of insulin is stimulation of glucose transport into muscle and adipose tissues. A defect in this system leads to the diabetes mellitus syndrome characterized by hyperglycemia, changes in the metabolism of carbohydrates, lipids and proteins, and by elevated incidence of vascular disorders. There are two main diabetes, insulin-dependent diabetes mellitus (IDDM) with an incidence of 1-43 per 100,000 inhabitants in the Western countries, and the non-insulin-dependent diabetes mellitus (NIDDM) whose incidence is between 100-800 per 100,000 inhabitants in the Western countries (The above reference).
Insulin is the main treatment of all IDDM and many NIDDM patients. Long-term treatment is predominantly based on subcutaneous administration of insulin formulations. There are long-, short- and intermediate-acting preparation which are used according to the special requirements of the patient. However, apart from the discomfort and troublesome feelings and the possibility of infection associated with daily injections along the entire lifetime (particularly with IDDM, formerly termed juvenile-onset diabetes mellitus), this kind of therapy has serious clinical problems mainly with the maintenance of the appropriate blood levels of the hormone resulting in non-physiological blood glucose levels and other complications. Although much effort has been made in developing insulin analogs [Brange, J; et al. (1990) Diabetes 13, 923-954] and genetic engineering methodologies [Sutherland, DER et al. (1989) Diabetes 38 Suppl 1, 46-54], there are no successful findings for solving the clinical problems associated with parenteral insulin injections.
One of the approaches aimed to cope with the aforementioned difficulty was to deliver the hormone non-invasively, via transdermal route of administration. By this procedure, the annoyance and inconvenience of the parenteral injections can be avoided, moreover, much steadier blood hormone levels can be achieved due to prolonged delivery of the drug. Several low molecular weight drugs have been formulated and are being clinically used as transdermal preparations. However, apart from a few medicines, many drugs, particularly peptides and proteins, are not successfully formulated for transdermal delivery. In vitro experiments have shown that .alpha.-melanocyte stimulating hormone analog can penetrate across human and mouse, but cannot penetrate rat skin [Dawson, B V et al. (1990) J. Invest. Dermatol. 94, 432-435; Dawson, B V et al. (1988) Life. Sci. 43, 1111-1117] and that enkephaiin can penetrate hairless mouse skin but in the presence of the enhancer n-decylmethyl sulfoxide and proteinase inhibitors [Choi, H K et al. (1990) Pharm. Res. 7, 1099-1106]. However, apart from one study with small number of mice which showed reduced levels of blood glucose after 4 hours of cutaneous application of insulin with enhancer [Liedtke, R K et al. (1990) Drug Res. 40, 880-883], no efficient in vivo transdermal penetration of peptides and proteins, by chemical means (e.g. enhancer or proteinase inhibitors) have been published. Transdermal penetration of various peptides and proteins can be enhanced by iontophoresis using electrical current for delivering charged agents across the skin. Various peptides and small proteins including insulin, calcitonin, vasopressin, luteinizing hormone-releasing hormone, (LHRH) leuprolide, thyrotropin-releasing hormone and cholecystokinin were tested in in vitro iontophoresis assays and some of them also in in vivo systems [Heit, M C et al. (1993) J Pharm Sci 1993 82(3):240-243; Srinivasan, V et al. (1990) J. Pharm. Sci. 79, 588-591; Burnette, R R and Marrero, D (1986) J. Pharm. Sci. 75, 738-743; Banga, A K and Chien, Y W (1993): Pharm. Res. 10, 697-702; Mao, X M et al. (1995) Yao. Hsueh. Hsueh. Pao. 30, 302-306; Mao, X M et al. (1995) Yao. Hsueh. Hsueh. Pao. 30, 881-885; Meyer, B R et al. (1989) Am. J. Med. Sci. 297, 321-325]. Additional technique to facilitate transdermal delivery of insulin by ultrasound vibration, termed sonophoresis, was used in both in vitro and in vivo systems [Tachibana, K and Tachibana, S (1991): J. Pharm. Pharmacol. 43, 270-271; Tachibana, K (1992) Pharm. Res. 9, 952-954; Mitragotri, S et al. (1995) Science. 269(5225), 850-853]. Although transdermal penetration of insulin and other proteins and peptides was enhanced by the sonophoretic and iontophoretic techniques, these procedures require complicated and an uneasy way of operation. Furthermore, the safety of long-term, daily use of this techniques was not confirmed. The fact that only one report, describing unsatisfactory results on the use of penetration enhancer in type II diabetic patients [Liedtke, R K et al. (1990): Drug Res. 40, 884-886] has been published, indicates the problematic issues of the above methods.